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10 Recommenciations SPACE TECHNIQUES An important part of general relativity remains completely untested: the prediction of gravitomagnetic effects, though exceedingly small in the solar system, should be checked experimentally. The National Aeronautics and Space Administration's (NASA's) Relativity Gyroscope Experiment (Gravity Probe B) is currently our best hope of detecting such an effect the dragging of inertial frames by the rotating Earth. The experiment calls for a level of technical sophistication not yet achieved in a spaceborne instrument. We are pleased to note that NASA has initiated the first phase of a two-stage program designed to accomplish this mission. ~ The highly successful use of solar-system ranging experiments to test general relativity should be continued. Ranging to planetary landers and orbiters has been particularly fruitful, and no such opportunities should be missed. The Mars Observer mission appears to be the first such opportunity if an accurate dual-frequency ranging system is included. It also is of great importance to keep improving the solar-system model with laser ranging to the Moon and radar ranging to the planets. These techniques are extremely cost-effective means for increasing the stringency of solar-system tests of general relativity. Two frontiers in gravitation research are the detection of gravita- 80

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RECOMMENDA TIONS 81 tional radiation and the testing of general relativity in second order. Promising ideas for space experiments in these areas should be encouraged and studied. Current concepts that warrant further study as possible future NASA missions are a long-baseline (~106-km) laser interferometer in solar orbit to detect gravitational waves in the important millihertz frequency range and a precision optical interferometer (POINTS) capable of testing relativistic light deflection by the Sun to second order. NASA currently has a proposal to send a precision clock into a near-Sun orbit (STARPROBE) to measure the gravitational redshift to second order, thus making a new (clock) measurement of the PEN parameter if. GROUND-BASED TECHNIQUES The strain amplitude sensitivity of interferometric gravitational- wave detectors can be increased by 2 or more orders of magnitude by the construction of baselines with lengths of ~5 km. This facility offers the opportunity for a breakthrough in gravitational-wave detection and should be pursued vigorously. Bar detectors are today the most sensitive gravitational-wave detectors. A diverse research program should enjoy continued support, with due attention being given to critical technologies. However, systematic observations should play an increasing role as a guide in the development of bar detectors. Pulsar observations have provided an impressive demonstration of gravitational-wave damping in the binary pulsar and significant upper limits for microhertz gravitational waves in the millisecond pulsar. Searches for and observations of pulsars and other compact objects, especially in binary systems, should be given high priority. ~ Laboratory experiments continue to play a role in gravitation research by testing with increasing precision the basic principles and predictions of gravitation theories. The fundamental nature of this work more than justifies its small cost. GRAVITATION THEORY Continued support for theoretical research is crucial to the health of gravitation physics. The essential prerequisites for a strong theory program are (a) support for a diversity of high-quality research areas, (b) availability of means for communication among theorists and also with scientists in other specialties, and (c) adequate opportunity for entry into the field by talented young people.

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82 GRA VI TA TION The strong relations of gravitation theory with other areas includ- ing particle theory, gravitation experiment, astrophysics, and pure mathematics are important to the field and should be fostered. Large-scale computation is playing an increasing role for certain problems in gravitation theory, as in many other fields. We welcome the initiatives currently under way to improve the access of physical scientists to supercomputers and smaller computers.